1. Field of the Invention
This invention pertains to electrical resistance compositions generally, and more specifically to compositions used in the manufacture of thick film cermet type resistors.
2. Description of the Related Art
Thick film resistor compositions for the purposes of this disclosure are defined as those compositions of one or more conducting materials combined with a binder. The binder may include plastics, glass compositions, ceramics, devitrifiable glasses and other materials to physically bind the conducting particles together. The binder may also assist in adhering the composition to a substrate.
In practice, the thick film material is applied to the substrate using one of a variety of well known techniques including but not limited to doctor blading, screen printing and tape transfer. Using one of these techniques, the thick film composition is applied to a substrate. Thereafter, the composition is typically heated or cured.
This disclosure pertains specifically to those thick film compositions that include a conductor component and a binder including both a transient component and a sinterable component. The transient component, sometimes referred to as a screening agent, may include such ingredients as oils, various resins, surfactants and other organic components. The binder usually includes a sinterable component.
The sinterable component may be composed entirely of a vitreous glass frit or may alternatively include ceramics and even ingredients which cause the devitrification of the glass into what is commonly referred to as a glass-ceramic. The word cermet is derived from a contraction of the words CERamic and METal.
In commercial use, these thick film compositions are usually applied to a ceramic or other heat resistant substrate, heated sufficiently to remove the transient binder materials and continued to be heated sufficiently to cause a sintering of the sinterable binder and fusion of the binder to the substrate. A high quality resistor capable of withstanding substantial temperatures and short term surges of power formed in this manner is of only moderate cost.
The need for high reliability and higher operating temperatures in a small low cost package fueled the development of these newer, more robust cermet materials. Exemplary of these materials is the ruthenium cermet materials illustrated in U.S. Pat. No. 3,304,199, assigned to the assignee of the present invention. This material, a ruthenium dioxide based material pioneered by the assignee and adopted worldwide as the industry standard yet today, offers a very high temperature material capable of surviving great extremes of power, temperature and environment.
The ruthenium cermet materials revolutionized the electronics industry and allowed applications never before possible. Unfortunately, these materials have as their primary conductive ingredients metals from the precious metal family. Most commonly used are ruthenium, silver, gold, palladium, and platinum. These materials offer several advantages over other alternatives, including the ability to be heated in air to the sintering temperature of the binder without degrading conductivity, resistance to environmental degradation, and, particularly in the case of silver, excellent conductivity.
Of course, these advantages are offset by the high cost and .limited availability of precious metals. Silver, one of the most affordable of the ingredients, is affected by moisture and can readily be induced to migrate. To reduce this migration, silver is often alloyed with palladium or platinum. However, palladium and platinum are much more expensive materials in the precious metal family. Additionally, the alloy has much worse electrical conductivity than silver alone.
There are many requirements for electrical resistors, one of which is cost. In addition, resistors are generally evaluated by their stability during and after adverse conditions such as temperature and humidity extremes, short duration power overloads (STOL--Short Term OverLoad), and even accelerated aging testing.
In addition, thick film resistor compositions will desirably have resistance values that are adjustable over wide ranges. This adjustability allows a manufacturer to stock and specify a few basic compositions, and then adjust the resistance values of the compositions for specific applications and production requirements.
The search for thick film materials offering ideal characteristics dates back about a century. As with most industries, no ideal combination of performance and price has been achieved. While the precious metal compositions pioneered by the present assignee and others have satisfied many of the performance requirements, the high cost of these materials continues to provide much impetus to developing lower cost materials of equivalent performance.
Base (non-noble) metal materials continue to find application, particularly where sufficient volumes exist to make the cost of materials a significant issue. However, existing base metal thick film cermet systems suffer several limitations. Among the limitations are available resistance ranges, blending characteristics, and processing restrictions.
Base metal resistor systems tend to be limited in resistance range where the material will still offer controlled TCR. TCR stands for Temperature Coefficient of Resistance, which is a measure of the amount of change in resistance over some temperature range. For the purposes of the remainder of this disclosure, TCR may be further divided into cold TCR (CTCR) and hot TCR (HTCR). Cold TCR is measured over the temperature range from -55 to +25 degrees Centigrade, while hot TCR is measured from +25 to +125 degrees Centigrade.
Resistivity for the purposes of this disclosure is measured in the units of ohms per square. This will be considered herein to be the resistance of a 1 mil thick film of equal length and width.
Typical tin oxide resistor systems may be formulated to offer from a few thousand ohms per square to several million ohms per square within a +-100 part per million per degree Centigrade (hereinafter ppm/.degree.C.) TCR. An example of tin oxide systems may be found in U.S. Pat. Nos. 4,655,965, 4,698,265, 4,711,803, and 4,720,418 assigned to the assignee of the present invention.
There are base metal resistor systems which offer lower resistance ranges with low TCR values. For example, titanium silicide may be formulated to offer resistance values from a few ohms per square to a few thousand ohms per square with +-100 ppm/.degree.C. TCR. This is illustrated in U.S. Pat. No. 4,639,391 assigned to the assignee of the present invention and incorporated herein by reference. Unfortunately, the titanium silicide formulation may not be blended together with the tin oxide formulation and still obtain low TCR values. Additionally, there are many applications requiring resistance values below 10 ohms per square. In this vein, there have been several attempts at providing low resistance base metal compositions.
U.S. Pat. No. 3,794,518 assigned to TRW illustrates the use of pre-alloyed copper nickel compositions to form thick film cermet resistors. The resistors created using these materials are very restricted in resistance range, typically in the tenths of an ohm per square range.
A similar composition using copper and nickel powders which alloy during the sintering process is illustrated in U.S. Pat. No. 5,037,670, assigned to the assignee of the present invention. Therein, copper nickel compositions are illustrated which may be varied using a number of techniques and which offer superior TCR control, but which are still limited to resistance values in the tenths of an ohm per square range.
Other base metal compositions of interest include lanthanum hexaboride, illustrated for example in U.S. Pat. No. 4,225,468 assigned to Du Pont, and also nickel chromium compositions such as illustrated in U.S. Pat. No. 4,060,663 assigned to TRW. The nickel chromium pre-alloy is disclosed therein from the examples to be limited to a restricted resistance range of from 1.8 to 23 ohms, depending upon variables such as alloy to glass frit ratios and upon firing temperatures. In practice, these ranges are much more restricted, due to inability to alter firing profiles during production, constraints on tolerance of variance in composition and contamination, and other similar issues.
While lanthanum hexaboride offers some advantage in terms of resistance range, control of the other performance characteristics is very difficult and resistance range is not complete. Further, lanthanum hexaboride is much more expensive.
What is still absent in the prior art is a wide range low TCR high performance base metal material. Much sought after are ways to come closer to achieving this ideal material.